OpenLDAP!

Univention Corporate Server is a pre-configured Linux server operating system free of charge. It's the best fit to provide Microsoft Server-like services on the cloud or on the premises, to run and operate corporate IT environments with Windows- and Linux-based clients and to extend those environments with proven enterprise software, also either on the cloud or on the premises.

FreeIPA is an integrated Identity and Authentication solution for Linux/UNIX networked environments. A FreeIPA server provides centralized authentication, authorization and account information by storing data about user, groups, hosts and other objects necessary to manage the security aspects of a network of computers.

The enterprise-class open source LDAP server for Linux. It is hardened by real-world use, is full-featured, supports multi-master replication, and already handles many of the largest LDAP deployments in the world. The 389 Directory Server can be downloaded for free, and set up in less than an hour using the graphical console.

Active Directory (AD) is a directory service that Microsoft developed for Windows domain networks and included in most Windows Server operating systems as a set of processes and services.

ApacheDS™ is an extensible and embeddable directory server entirely written in Java, which has been certified LDAPv3 compatible by the Open Group. Besides LDAP it supports Kerberos 5 and the Change Password Protocol. It has been designed to introduce triggers, stored procedures, queues and views to the world of LDAP which has lacked these rich constructs.

JumpCloud®, the first Directory-as-a-Service® (DaaS), is Active Directory® and LDAP reimagined. JumpCloud securely manages and connects employee identities to IT resources including devices, applications, storage file servers and networks. JumpCloud is a central source of authentication, authorization, and management of employees and all to which they require access. It is your only next gen directory services. Cloud-based alternative to Microsoft Active Directory or LDAP.

It's an active directory replacement built on CentOS + Samaba4. Includes support for Directory, DNS, DCHP, and NTP type services.